1. Field of the Invention
The present invention relates generally to preamplifier circuits, and more particularly to preamplifier circuitry with signal interference cancellation suitable for use in magnetic storage devices.
2. Description of the Related Art
A magnetic storage device (such as a disk drive) typically includes a magnetic disk, a spindle motor which spins the magnetic disk, a magnetic head which has one or more read sensors, a read/write integrated circuit (R/W IC), and a suspension interconnect coupled between the read sensor and the R/W IC. The read sensor, such as a magnetoresistive (MR) sensor or a giant magnetoresistive (GMR) sensor, for example, is used for reading data from the disk. The read sensor is coupled to an input of the R/W IC, which generally includes read signal processing circuitry. The read signal processing circuitry biases the read sensor with a fixed direct current (DC) bias voltage or current, amplifies signals read from the disk, and may provide further processing of the amplified signals. The read sensor is coupled to the R/W IC through the suspension interconnect, which includes electrical conductors (e.g. copper alloy traces) primarily carried along an actuator arm.
The resistance of such read sensors changes in response to changing magnetic flux orientations on the magnetic disk. Changes in resistance of the read sensor translate into a varying analog electrical signal which is received and processed by the R/W IC. The processed analog signals are ultimately converted into digital data. In this general fashion, the magnetic storage device is able to read data from the disk at relatively high data rates (e.g. greater than 500 megabits per second (Mbs)). Unfortunately, without appropriate preamplifier circuitry in the read circuitry, too much interference may be picked up while reading the signals from the read sensor to the input of the read amplifier. The preamplifier circuitry typically amplifies low-level differential-mode readback data signals along with undesirable common-mode interference signals. Such interference ultimately affects the accuracy of the signals being read from the disk.
Interference signals are introduced from several different sources in the disk drive system. One such source is electronic circuitry that drives the spindle motor. Another source arises in the application environment for disk drives, such as Personal Computers (PCs), servers, and metal-rack supports. Finally, transmission line interference-coupling effects of the suspension interconnect, during high data rate operation, may undesirably influence the spectral content of the read signal.
One known solution for reducing common-mode interference signals in the read circuitry is the use of a differential circuit preamplifier. Differential circuits, however, have several drawbacks. For one, they consume a relatively large amount of power. Also, they occupy a relatively large area on the R/W IC. Finally, due to “floating inputs” (i.e. controlled near-ground potential), the front-end circuit has a relatively long transient recovery time. The following patents disclose the selection of a single read port and/or are differential circuit topologies: U.S. Pat. Nos. 5,859,564; 6,175,462; 5,323,278; and 5,444,579.
What is needed is an improved preamplifier circuit which reduces interference signals, especially in a disk drive environment.